Oil refinery cracking processes allow the production of light products such as liquified petroleum gas (LPG) and gasoline from heavier crude oil distillation fractions such as gas oils and residues. Current cracking technologies can be classified into the two general categories: thermal cracking (also known as steam cracking) and catalytic cracking. Specifically, Fluid Catalytic Cracking (FCC) is a conversion process in a refinery for upgrading low value heavy hydrocarbons into high value light distillates and LPG. The process employs solid acid catalysts such as zeolites to promote cracking reactions in a riser reactor/fluid bed regenerator system.
FCC catalysts contain zeolites as an active component. Such catalysts generally take the form of small particles containing both an active zeolite component and a non-zeolite component. Frequently, the non-zeolitic component is referred to as the matrix for the zeolitic component of the catalyst.
In FCC catalysts, the active zeolitic component is typically incorporated into the catalyst by one of two general techniques. In one technique, the zeolitic component is crystallized and then incorporated into matrix in a separate step. In the second technique, an in-situ technique, particles are first formed and the zeolitic component is then crystallized in the particles themselves to provide FCC catalysts containing both zeolitic and non-zeolitic components.
Two products that are particularly undesirable in commercial catalytic cracking processes are coke and hydrogen. Even small increases in the yields of these products relative to the yield of gasoline can cause significant practical problems. For example, increases in the amount of coke produced can cause undesirable increases in the heat that is generated by burning off the coke during the highly exothermic regeneration of the catalyst. Conversely, insufficient coke production can also distort the heat balance of the cracking process. In addition, in commercial refineries, expensive compressors are used to handle high volume gases, such as hydrogen. Increases in the volume of hydrogen produced, therefore, can add substantially to the capital expense of the refinery.
Improvements in cracking activity and gasoline selectivity of cracking catalysts do not necessarily go hand in hand. Thus, a cracking catalyst can have outstandingly high cracking activity, but if the activity results in a high level of conversion to coke and/or gas at the expense of gasoline the catalyst has limited utility. Catalytic cracking activity in present day FCC catalysts is attributable to both the zeolite and non-zeolite (e.g., matrix) components. Zeolite cracking tends to be gasoline selective. Matrix cracking tends to be less gasoline selective.
Recently, however, FCC apparatus have been developed which drastically reduce the contact time between the catalyst and the feed which is to be cracked. Conventionally, the reactor is a riser in which the catalyst and hydrocarbon feed enter at the bottom of the riser and are transported through the riser. The hot catalyst effects cracking of the hydrocarbon during the passage through the riser and upon discharge from the riser, the cracked products are separated from the catalyst. The catalyst is then delivered to a regenerator where the coke is removed, thereby cleaning the catalyst and at the same time providing the necessary heat for the catalyst in the riser reactor. The newer riser reactors operate at lower residence time and higher operating temperatures to minimize coke selectivity and delta coke. Several of the designs do not even employ a riser, further reducing contact time to below one second. Gasoline and dry gas selectivity can improve as a result of the hardware changes. These FCC unit modifications are marketed as valuable independent of the type of catalyst purchased, implying an absence of systematic problems in state of the art catalyst technology.
The processing of increasingly heavier feeds in FCC type processes and the tendency of such feeds to elevate coke production and yield undesirable products has also led to new methods of contacting the feeds with catalyst. The methods of contacting FCC catalyst for very short contact periods are of particular interest. Thus, short contact times of less than 3 seconds in the riser, and ultra short contact times of 1 second or less show improvements in selectivity to gasoline while decreasing coke and dry gas production. However, higher contact times are more prevalent in older FCC units where the times can be up to 7 seconds.
To compensate for the continuing decline in catalyst to oil contact time in FCC processing, the “equilibrium” catalysts in use are tending to become more active. Thus, attempts at increasing the total surface area of the catalyst are being pursued and as well, the level of rare earth oxide promoters added to the catalysts is increasing. Moreover, cracking temperatures are rising to compensate for the reduction in conversion. Unfortunately, the API gravity of the bottoms formed during short contact time (SCT) often increases after a unit revamp, leading some to suggest that the heaviest portion of the hydrocarbon feed takes longer to crack. Further, while a high total surface area of the catalyst is valued, the FCC process still values attrition resistance.